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IC 8976 



Bureau of Mines Information Circular/1984 




Geologic Factors in Coal Mine Roof 
Stability— A Progress Report 

By Noel N. Moebs and Raymond M. Stateham 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8976 



Geologic Factors in Coal Mine Roof 
Stability— A Progress Report 

By Noel N. Moebs and Raymond M. Stateham 



UNITED STATES DEPARTMENT OF THE INTERIOR 
William P. Clark, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 




Library of Congress Cataloging in Publication Data: 



Moebs, Noel N 

Geologic factors in coal mine roof stability. 

(Information circular ; 8976) 

Bibliography: p. 25-27. 

Supt. of Docs, no.: I 28.27:8976. 

L Coal mines and mining— Safety measures. 2. Mine roof control. 
3. Coal— Geology. I. Stateham, Raymond M. II. Title. HI. Series: 
Information circular (United States. Bureau of Mines) ; 8976. 



TN295.U4 622s [622\334] 84-600018 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Background 3 

Early investigations of coal mine roof instability 3 

Geologic approaches 3 

Recent Bureau studies 5 

Review of contract research on the structural and lithologic character of coal 

mine roof rock 6 

Developing geological structural criteria for predicting unstable mine roof 

rocks , contract H0133018 6 

Significant findings 6 

Comment 8 

Methods and criteria for producing a photographic core logging manual for the 

Pittsburgh Basin, contract J0188115 8 

Significant findings 9 

Comment 9 

A study of roof falls in underground mines, contract H0230028 9 

Significant findings 10 

Comment 11 

Premining identification of hazards associated with coal mine roof measures, 

contract J0177038 11 

Significant findings 11 

Comment 12 

Engineering study of structural geologic features of Herrin (No. 6) coal and 

associated rock in Illinois, contract H0242017 12 

Significant findings 13 

Comment 13 

Review of contract research on the physical properties of coal mine roof rock. . 14 

Failure of roofs in coal mines , contract H0232057 14 

Significant findings 15 

Comment 16 

Effects of temperature and humidity variations on stability of coal mine roof 

rocks, contract H0122111 16 

Significant findings 16 

Comment 17 

Correlation of mine roof failure with time elapse before support installa- 
tion, contract HOI 11413 17 

Significant findings 18 

Comment 18 

Cause and prevention of failure of freshly exposed shale and shale materials 

in mine openings, contract 60111809 18 

Significant findings 18 

Comment 19 

Control of shale roof deterioration with air tempering, contract J0188028.... 19 

Significant findings 19 

Comment 20 

Conclusions regarding geologic structure and lithologic character of mine roof. 21 

Roof rolls 21 

Slickensides 21 

Interlaminated shale-sandstone-coal 22 

Joints 22 



<=> 



ii 



CONTENTS— Cont inued 

Page 

Clay dikes 22 

Core manual and hazard map 24 

Conclusions regarding physical properties of coal mine roof rock 24 

Roof disintegration and humidity 24 

Time lapse before roof support installation 25 

References 25 

ILLUSTRATIONS 

1. Sandstone-filled paleochannel or roof roll 4 

2 . Common forms of clay veins or clay dikes 5 

3 . Slickensides in claystone roof strata 5 

4 . Roof roll with heavily slickensided margins 7 

5. Thinly interlaminated, poorly bonded sandstone, shale, and coal roof 

strata 7 

6 . Low splay sandstone zone in mine roof 10 

7. Composite hazard zone map 12 

8. Roof rolls in the Herrin Coalbed 14 

9. Claystone dike or clay vein transecting coalbed and shale roof 15 

10. Shear- or "cutter"-type roof failure developing along rib line 23 





UNIT OF MEASURE ABBREVIATIONS USED 


IN 


THIS REPORT 


°F 


degree Fahrenheit pin/in 




microinch per inch 


ft 


foot pet 




percent 


h 


hour psi 




pound (force) per 
square inch 


mi 


mile 








yr 




year 


min 


minute ' 







GEOLOGIC FACTORS IN COAL MINE ROOF STABILITY-A PROGRESS REPORT 

By Noel N, Moebs and Raymond M. Stateham 



ABSTRACT 

This report summarizes 10 selected Bureau of Mines research contract 
reports produced from 1970 to 1980 which consist largely of geologic 
studies of coal mine roof support problems. Significant highlights 
from the contract final reports are discussed and presented in practi- 
cal terms. The selected reports focus on the Appalachian and Illinois 
coal mining regions. 

In the Appalachian coal region, two geologic structures, roof rolls 
and slickensides, predominate over all structures as features that di- 
rectly contribute to roof falls. Studies of these and other structures 
are reviewed, and improved methods of utilizing drill core and core 
logs to prepare hazard maps are presented. Among the reports described 
are several on the weakening effects of moisture on shale roof, as de- 
termined from both laboratory and underground measurements , and an as- 
sessment of air tempering as a humidity-control method. Also summar- 
ized are findings concerning the time lapse between roof exposure and 
permanent support installation as a factor in the effectiveness of roof 
bolting. 



1 Geologist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 
2 Geophysicist, Denver Research Center, Bureau of Mines, Denver, CO. 



INTRODUCTION 



Instability in coal mine roofs is con- 
sidered to be the result of one or both 
of two basic types of causative factors — 
defect-related and/or non-defect-related. 
Defect-related causes are geologic by 
definition because they are related to 
naturally occurring features in the 
rock mass around the mine opening. Non- 
defect-related conditions can be de- 
scribed by one word — stress. The stress 
exists because of geostatic pressure, re- 
gional or local geologic conditions, or 
the creation of the mine opening. This 
report deals with several investigations 
of defect-related geologic factors and 
their contributions to instability in 
coal mine roofs. It is anticipated that 
a better understanding of geologic phe- 
nomena such as structure and lithology as 
contributing factors to instability will 
lead to a keener insight into the causes 
of roof falls. This better understanding 
should also lead to improved methods of 
roof support, and techniques for the pre- 
diction and classification of potentially 
unstable roof in advance of mining. Con- 
sequently, the Bureau of Mines is identi- 
fying and describing geologic features 
that contribute to roof instability, 
through research grants and contracts 
with educational and consulting organiza- 
tions and through the work of Bureau re- 
searchers working in cooperating mines. 

The emphasis in this summary report is 
on contract reports written since 1970, 
when the Bureau renewed its efforts to 
resolve the interrelation between geology 
and roof conditions. Reports from other 
sources are cited where pertinent. It is 
hoped that this annotated summary will 
provide mine engineers and geologists 
with useful information which otherwise 
might remain obscured in technical re- 
ports that do not receive wide circula- 
tion. Although this review of geologic 
studies concentrates on the Appalachian 
and Illinois coal mining regions, some 
topics discussed might be applied to 
other localities as well. It is also 



hoped that this report will encourage 
more mining companies to document geo- 
logic features in active mines and in 
zones where unusually heavy support is 
required. 

The urgent need to use all available 
means to deal with roof support problems 
is made clear by the recurring accidents 
that result from roof falls. These acci- 
dents result in fatalities, injuries, and 
costs to operations in terms of lost la- 
bor time, compensation, delays, cleanup, 
and repairs. Up to and including 1937, 
roof falls accounted for about 45 pet of 
the total number of fatalities in under- 
ground coal mines (14) 3 and in 1970 roof 
falls still accounted for 41 pet of the 
fatalities. In 1977, a total of 1,420 
injuries from roof falls was reported to 
the Mine Safety and Health Administration 
(MSHA), of which 30 were fatal. The fol- 
lowing tabulation of total fatalities 
from roof falls for recent years shows 
that this hazard continues to constitute 
a serious problem in coal mines: 

1978 33 

1979 65 

1980 32 

1981 41 

1982 52 

Falls in which there is no injury are in- 
numerable. These falls commonly block 
haulageways or air courses and damage 
track, cables, and conveyor belts. The 
geologic studies reviewed in this report 
are part of a renewed effort toward ad- 
vancing the technology needed to make 
coal mining safer and more productive 
through improved roof control. 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



BACKGROUND 



EARLY INVESTIGATIONS OF COAL 
MINE ROOF INSTABILITY 

The problem of coal mine roof support 
has been the object of numerous investi- 
gations in the United States since early 
in the 20th century. A Bureau of Mines 
report (14) on roof movement in coal 
mines lists 62 references published be- 
tween 1912 and 1937. In that report, 
however, a failure of coal mine roof was 
commonly described and explained in the 
context of a particular locality or hy- 
pothetical case. This practice has been 
continued to the present , with the geo- 
logic environment receiving little atten- 
tion. Unfortunately, emphasis upon the 
unique nature of a roof fall has made it 
difficult to isolate common causes of 
roof instability, and the role of spe- 
cific geologic phenomena is little under- 
stood. Meanwhile, roof failure continues 
to be the major hazard in underground 
mining. 

Since 1930, great advances have been 
made in the field of rock mechanics and 
coal mine roof support technology, as 
summarized by Adler and Sun (_1) . A study 
reported by the Bureau in 1941 (15) con- 
cerned the effect of moisture on mine 
roof disintegration. 

GEOLOGIC APPROACHES 

The first significant report on the 
geologic characteristics of coal mine 
roof rock in the Eastern United States 
was not published until 1948 when Holland 
(17) described his observations for the 
West Virginia Coal Mining Institute. 
This was followed shortly by a somewhat 
more detailed discussion by Price (25) on 
roof rock geology and its influence on 
selecting types of roof support. From 
1960 to 1972, publications dealing with 
geologic features that affect coal mine 
roof stability appeared in greater num- 
bers. These include three of special 
significance for their contributions to 
the understanding of roof support prob- 
lems. In the first, by Diessel and 



Moelle (8), sedimentary and structural 
features in Australian floor and roof 
strata were analyzed to determine their 
influence on the stability of mine open- 
ings, and the methods used to analyze 
these features were outlined. Diessel 
and Moelle concluded that both sedimen- 
tary and structural analyses were useful 
in planning future workings of a mine 
because the analyses could be used to 
establish the trends of troublesome geo- 
logic structures such as faults, wash- 
outs, and rolls. 

The second contribution of special sig- 
nificance was by Weir (34). Although 
brief, it is an excellent summary of geo- 
logic conditions and related roof falls 
encountered in Sullivan County, IN. 
Weir's conclusions, which caution against 
oversight of geologic detail, are worthy 
of quoting in their entirety: 

In evaluating roof rock condi- 
tions, the geologist must look at 
the rocks from many viewpoints. No 
single physical or chemical parame- 
ter tells the whole story. Not on- 
ly are the lateral variations in 
the rocks important but vertical 
variations are also. Not only may 
the rock be significantly different 
from one foot to the next but, in 
some cases, from one centimeter to 
the next. 

Lithology and thickness of beds, 
jointing, strength of bedding plane 
bond, and the effect of mois- 
ture are important considera- 
tions. No single criterion seems 
to be adequate for practical roof 
evaluation. 

In the third significant publication, 
Dahl and Parsons (_7) combined finite- 
element stress analysis with a conven- 
tional examination of geologic factors 
affecting roof fall severity. They em- 
phasized the importance of the geometry 
of a situation, room width, and residual 
compressive ground stresses. 



An additional publication edited by 
Donaldson (9), appearing in 1969 for use 
as a guidebook, is a comprehensive de- 
scription of the formation of the Permo- 
Carboniferous rocks in the Appalachian 
Basin; it contains abundant information 
directly applicable to coal mining and 
exploration. Also included are several 
separate sections in which the geologic 
setting, as specifically related to roof 
rock conditions, is discussed. More re- 
cently, Overbey (24) described procedures 
for constructing a hazard-potential map 
using a wide range of geologic features 
which have been cited as contributing 
factors in roof falls. In 1975, McCul- 
loch, Diamond, Bench, and Deul (20) re- 
viewed geologic factors affecting mining 
of the Pittburgh Coalbed; and in 1979, 
Stahl (29) similarly described geologic 
features affecting coal mine roof in 
general. 



The Bureau has been actively and con- 
tinuously investigating coal mine roof 
instability since it was established in 
1910 ( 14) and has issued numerous techni- 
cal and educational papers on improving 
methods of roof support. However, the 
factor of geologic variables and their 
bearing on the effectiveness of roof sta- 
bility has not been fully appreciated 
until recently, and techniques for ana- 
lyzing these variables are only now 
emerging. It is at this stage that the 
Bureau is attempting to screen the numer- 
ous geologic investigations to identify 
those that seem to have the greatest po- 
tential for application to the study and 
prevention of coal mine roof failure. 
Among the more promising geologic ap- 
proaches are the conventional ones used 
in several of the reported contract 
studies whereby roof falls are related to 
mappable geologic features. 



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Scale, ft 



FIGURE 1. - Sandstone-filled paleochannel or roof roll. 



RECENT BUREAU STUDIES 

Beginning about 1970, the Bureau re- 
newed its efforts to identify the geo- 
logic factors that contribute signifi- 
cantly to roof failure. The initial 
effort consisted of an investigation of 
roof falls in the northern portion of the 
Pittsburgh Coalbed in southwestern Penn- 
sylvania and northern West Virginia. 
This work included numerous interviews 
with operating personnel and examination 
of the roof in virtually every major mine 
in the study area. Based on these activ- 
ities and analyses of roof fall maps, it 
was concluded that the incidence of falls 
was significantly higher beneath stream 
valleys, in close proximity to paleochan- 
nels (fig. 1), clay veins (fig. 2) and 
other small-scale geologic features, and 
wherever large slickensides (fig. 3) oc- 
curred in the roof strata. Falls also 
commonly occurred where roof rock con- 
sisted of weak claystones or laminated 
sandstone. Broad gentle folds, charac- 
teristic of the Allegheny Plateau, showed 
no relation to roof fall zones. These 
results confirmed that geologic features 
contribute significantly to roof failure 
and should be studied in greater detail. 
Therefore, the Bureau awarded research 
contracts covering a variety of ap- 
proaches to the problem of roof failure. 
Although the geologic studies dominated, 
studies of interrelated engineering and 
mining conditions were included in these 
investigations. In several instances 
where the investigation began as an 
engineering-type study, the geologic con- 
ditions soon emerged as the dominating 
factor. 

From 1970 to 1980, 10 research con- 
tracts were awarded by the Bureau in 
which the role of geology in roof failure 
was the main theme. In the following 
sections of this paper, the significant 
findings of these contracts are reviewed, 
and the results of relevant research con- 
ducted by the Bureau are also cited. Be- 
cause of the widely divergent and some- 
times interrelated topics included in 
these contracts, any attempt at strict 
classification would be futile. However, 
the contract studies are grouped here 



Scale, ft 
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FIGURE 2. - Common forms of clay veins or 
clay dikes. 





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i i_ 



Scale, ft 



FIGURE 3. - Slickensides in claystone roof strata. 



into two broad categories: those dealing 
primarily with the structural and litho- 
logic character of mine roof rock and 
those relating to physical properties of 
roof rock, including changes in these 
properties upon exposure. 



The complete texts of these contract 
reports can be examined at Bureau librar- 
ies where they are held on open file, 4 or 
copies can be purchased from the National 
Technical Information Service (NTIS), 
Springfield, VA 22161. 



REVIEW OF CONTRACT RESEARCH ON THE STRUCTURAL AND LITHOLOGIC 
CHARACTER OF COAL MINE ROOF ROCK 



DEVELOPING GEOLOGICAL STRUCTURAL CRITERIA 
FOR PREDICTING UNSTABLE MINE ROOF ROCKS, 
CONTRACT HO 1330 18 (1_8) 

This study was designed to identify 
and describe the geologic structures re- 
lated to roof falls in the Highsplint and 
Bailey Creek Mines in eastern Kentucky. 
The main objective was to develop methods 
for predicting and classifying roof con- 
ditions on the basis of geologic studies. 

Both mines are located in Harlan County 
about 1.5 mi east of the Pine Mountain 
thrust fault and in the Darby and Harlan 
Coalbeds of the over thrust block. Depth 
of overburden ranges from near zero to 
1,500 ft beneath mountain tops. Neither 
mine had been core drilled; therefore, 
the investigation was conducted largely 
by detailed underground mapping and sam- 
pling, followed by laboratory studies of 
roof rock samples, examination of limited 
outcrop exposures, and joint analysis. 

Significant Findings 

Field studies showed that roof support 
problems in the Highsplint Mine (Darby 
Coalbed) were related to either of two 
geologic conditions. The first was large 
rolls, or undulations, in the basal con- 
tact of thick sandstone (fig. 4) occur- 
ring within 15 ft of the roof and trun- 
cating the horizontal shale strata, with 
intensely slickensided shale close to the 
rolls. The second condition was mine 
roof which consisted (in the first few 
feet) of lenticular, thinly inter lami- 
nated, poorly bonded sandstones and shale 
(fig. 5). The rolls were U-shaped, 
strongly linear, and about 20 ft wide. 
The interval of roof rock between the 
coalbed and the main sandstone roof 
ranged from to 15 ft due to the roll- 
ing, or undulatory, basal contact of the 



sandstone. The rider coal was often re- 
placed by the sandstone. Jointing in the 
main sandstone roof was not a major fac- 
tor in roof failure. 

At the Highsplint Mine, the directional 
trends of the major sandstone roof rolls 
were established by underground mapping 
and successfully projected from one set 
of mine entries to another , over dis- 
tances that ranged from 400 to almost 
2,000 ft. Projection of the geologic 
trends was facilitated by classification 
of mine entry roof according to four pre- 
dominant geologic conditions , as follows : 



1. Laminated 
slickensides. 



shale with diagonal 



2. Laminated shale with horizontal or 
low-angle slickensides. 

3. Rolling base of thick sandstone 
above roof level, with intervening shale 
between rolls. 

4. Rolling sandstone at roof level. 

Through examination of the coalbed and 
roof rock exposed along the crop line 
surrounding a portion of the mine, it was 
established that if well-defined sand- 
stone rolls could be detected in outcrop, 
roof problems related to the rolls could 
be expected to occur in the mine. 

In contrast, roof failure in the Bailey 
Creek Mine (Harlan Coalbed) was attri- 
buted to lenses of rippled, cross bedded, 
and flaggy laminated sandstone that cut 
across the shale roof strata. These 

4 Libraries are located at Bureau fa- 
cilities in various cities. Not all 
open file reports are available at all 
libraries. 




FIGURE 4. - Roof roll with heavily slickensided margins. 




FIGURE 5. • Thinly interlaminated, poorly bonded sandstone, shale, and coal roof strata. 



lenses ranged from 60 to 100 ft wide and 
are probably flood plain splays. Roof 
falls commonly extended 12 to 15 ft 
upward to the rider coal. The occur- 
rence of roof cutters or shears at the 
intersection of roof and rib indicated 
that high stresses were responsible for 
some roof failure. Also, some joint- 
related falls occurred near the crop line 
and beneath some stream valleys where the 
overburden was 200 ft or less. The pre- 
ferred direction of jointing under shal- 
low cover was used to plan a proposed set 
of entries in order to minimize the de- 
trimental effects of driving parallel to 
closely spaced jointing. 

A study was also conducted on the use 
of Landsat (satellite) imagery for the 
detection of linear zones of structural 
weakness that might contribute to unsta- 
ble roof conditions. It was concluded 
that Landsat imagery provides a rapid and 
accurate means of determining the loca- 
tion and trend of major geologic struc- 
tures. It also helped in tracing sig- 
nificant structures from one mine to 
another. In the northern West Virginia 
area of the Dunkard Basin, the study 
showed a close correlation of Ronchi 
grating directions and the roof fall 
zones occurring under stream valleys, 
commonly a north-south trend. This sug- 
gests that in situ stresses are an impor- 
tant factor in this mode of failure. 

Comment 



A major effort in terms of worker-hours 
was expended in mapping the character and 
trend of rolls, sandstone channels, and 
fractures in main roof that were causally 
related to roof falls. Projections of 
these geologic features were used to 
identify potentially hazardous zones , and 
new development was planned to avoid 
these zones wherever possible. Because 
of the discontinuous and erratic nature 
of most roof structures, projections be- 
yond a few hundred to a thousand feet are 
unreliable. However, mapping of well- 
defined linear roof structures may be 
sufficently simplified that an occasional 
underground visit by a geologist will be 
adequate to record the structures on 



operations maps and serve as an aid in 
laying out development. 

The classification of roof areas de- 
vised in this study requires some geo- 
logic expertise, but the technique aids 
in mapping trends of roof structures and 
may prove useful in determining the prob- 
able support requirements in areas to be 
mined . 

A simplified method was devised in 
which Landsat film transparencies were 
used for direct plotting of lineaments to 
scale for correlation with surface and 
subsurface data. The method proved use- 
ful in tracing structures from one mine 
to another. 

METHODS AND CRITERIA FOR PRODUCING 
A PHOTOGRAPHIC CORE LOGGING MANUAL 
FOR THE PITTSBURGH BASIN, 
CONTRACT JO 188 115 (13) 

The purpose of this contract was to de- 
velop and produce a field guidebook for 
the identification and classification of 
cored roof rock in the northern Appala- 
chian bituminous coal region. The spe- 
cific objective was to provide drillers, 
engineers, and geologists with the means 
to standardize the identification of core 
roof rock so that drill-core data can be 
more fully utilized in mine planning and 
ground control. Drill-core data can be 
applied to mapping areas of potentially 
bad roof conditions, determining optimum 
roof support methods, and establishing 
the continuity of a coalbed. 

Preparation of the guidebook was accom- 
plished by collecting, sorting, classi- 
fying, and photographing numerous samples 
of core from throughout the northern 
Appalachian coal region. Representative 
core samples for each of the major rock 
types were photographed in color both 
in wet and dry condition. A descriptive 
key was developed for each major rock 
type and referenced to the appropri- 
ate color photographs for ease in identi- 
fying core samples. Code numbers were 
assigned to each rock type for the pur- 
pose of computer storage, manipulation, 
and retrieval of data. The guidebook is 



softbound, printed on weatherproof paper, 
and measures 5-1/2 by 8-1/2 in. 

The same contractor (University of 
South Carolina) produced a similar guide- 
book under Bureau contract H0230028 (12) 
for cored rocks from the Pocahontas Basin 
in southern West Virginia, eastern Ken- 
tucky, and southwestern Virginia. 

Significant Findings 

The preparation of this drill-core 
guidebook demonstrated that a practical 
system of standardized classification can 
be developed even in regions where coal 
measure rocks are commonly gradational 
both vertically and laterally. 

Comment 

Adequate exploratory drill-core iden- 
tification is fundamental to sound geo- 
logic analysis of the core log data. The 
preparation of isopach maps, geologic 
sections, coal reserve maps, and mine 
layouts depends on the availability of 
properly classified drill-core data, 
which can be achieved through the use of 
this guidebook. 

The major rock types illustrated and 
described in the guidebook can be easily 
recognized by the core driller or geolo- 
gist; so in all drill-core logs based on 
this system, the rock nomenclature will 
be standard. Thus, drill core logged by 
different individuals should be classi- 
fied identically and should be fully 
comparable. 

Standardized classification and coding 
of drill core is especially essential for 
the successful use of computer-generated 
maps of various types; such maps are rap- 
idly gaining in popularity and are easily 
obtained once precise drill-hole data are 
acquired. Once the process of computer 
map generation has begun, however, the 
judgment of the geologist is not utilized 
until completion. 



A STUDY OF ROOF FALLS IN UNDERGROUND 
MINES, CONTRACT H0230028 (_12) 

The objective of this study was to 
identify the major geologic features oc- 
curring in mine roof that lead to roof 
failure. In particular, the purpose was 
to establish through statistically valid 
analysis specific rock types, structures, 
rock sequences, or other attributes that 
relate to roof quality. The investi- 
gation was performed over a period of 
4 yr in the Pocahontas No. 3 Coalbed of 
southern West Virginia and southwestern 
Virginia. 

Five operating underground mines were 
selected for detailed study. In each 
mine, investigators documented geologic 
factors in the roof that related to roof 
falls. The mines were selected on the 
basis of statistical analyses of the 
range in variation in mining conditions, 
roof conditions, and geologic phenomena. 
Within each mine, roof characteristics 
were documented in areas designated as 
good and bad roof according to stability. 

The sequence and lateral distribution 
of roof rock types were determined from 
drill-core records. Roof structures were 
identified and mapped in each mine. Min- 
ing practices related to roof support 
were documented. 

The study concluded with quantitative 
determinations of (1) the specific rock 
types , sequences , structures , and charac- 
teristics, that correlated directly with 
roof quality and (2) the extent of rocks 
having certain roof attributes. 

During the part of the study in which 
drill core and drill-core logs were ana- 
lyzed, it became evident that an improved 
method of core identification and logging 
was necessary to assure uniform use 
of rock nomenclature. Therefore, a pro- 
totype field guidebook was developed to 
facilitate logging and classification; 
it included a key to rock types and 



10 



color photographs of drill core. (The 
guidebook developed for this study was 
the prototype for the drill-core guide- 
book, described in the preceding section.) 

Significant Findings 

As a result of this investigation, sev- 
eral relationships between roof quality 
and roof rock became relatively clear; 
these relationships are listed below. 

1. Thick shale — that is, a solid mass 
of shale occupying the entire 40 ft above 
the top of the coalbed — produces the best 
quality and most stable roof. 

2. Thick sandstone — that is, a solid 
or nearly solid mass of sandstone occupy- 
ing the entire 40 ft above the top of 
the coalbed — may produce either good or 
bad roof. Poor roof occurs when shale, 



ironstone pebbles, coal stringers, or 
crossbedding are abundant in the lower 
3 or 4 ft of the sandstone. 

3. Low splay sandstone (within 8 ft 
above the top of the coalbed; figure 6) 
and high splay sandstone (8 to 30 ft 
above the top of the coalbed) may produce 
either good or bad roof conditions, de- 
pending on other factors. 

4. Roof strata that become more 
coarsely grained upward, as in a shale- 
sandy shale-sandstone sequence, produce 
excellent roof when the entire sequence 
is about 30 or more feet thick. 

5. Various types of clays tone, common- 
ly known as fire clay, seat earth, clod 
rock, or under clay, are among the poorest 
quality roof materials when they are mas- 
sive and unlaminated. 



Shale 



"V 



Splay sandstone 



Bolts 



Z^S 



2 



:V 





Entry 



- Underclay 



o 



5 

_l_ 



Scale, ft 



10 

J 



FIGURE 6. - Low splay sandstone zone in mine roof. 



11 



6. Slickensided rock produces the 
worst roof problems. Slickensided struc- 
tures form as shear planes in a number 
of different rock types and geologic 
settings, but are most commonly found 
in claystone, at the contact between 
channel-scour sandstones and shale, and 
surrounding kettlebottoms and strongly 
inclined slump deposits. 

Comment 

The importance of this work lies in 
the systematic manner in which various 
types of roof were categorized and 
related to roof competence. The six 
basic types of roof described above can 
be recognized in accurately prepared 
drill-core logs , thus enabling the engi- 
neer or geologist to block out reserve 
areas according to anticipated roof com- 
petence and probable method of optimum 
roof support. The precision of this 
method of roof assessment rests largely 
on the spacing of exploratory drill 
holes. 

The prototype guidebook to cored rock 
made it possible to accurately identify 
and classify the roof strata. 

PREMINING IDENTIFICATION OF HAZARDS 
ASSOCIATED WITH COAL MINE ROOF 
MEASURES, CONTRACT J0177038 (33) 

The objective of this contract was to 
summarize in a graphic manner (on maps) 
all rock fall hazards related to geologic 
conditions in the Pittsburgh Coalbed for 
a nine-county area centering around the 
northern panhandle of West Virginia. The 
mapped area included portions of eastern 
Ohio, northern West Virginia, and south- 
western Pennsylvania. 

The study continued for about 18 months 
and was divided into two phases. Phase 1 
included data collection from drill-core 
logs , mine maps , interviews , roof fall 
reports , published technical reports , and 
mine visits. Statistical analysis of the 
data was used to identify 12 geologic 
and mining variables that are causally 
related to hazardous roof conditions, 
and a hazard-classification system was 



developed for use in preparing hazard 
zone maps. Each variable was weighted in 
terms of potential roof fall occurrence 
and severity. 

In phase 2, a series of overlay maps on 
a scale of 1:62,500 was compiled, illus- 
trating the occurrence of geologic vari- 
ables ; and a composite hazard zone map 
was generated by machine processing and 
integration of the variables. All maps 
were bound into five separate folios 
28- by 36-in indexed by county. The 
maps were developed using methods that 
facilitate user modification as more 
site-specific information becomes avail- 
able or when selected variables are 
weighted in different order. A final 
technical report and map users' guide 
were written to accompany the folio. 

Significant Findings 

The systematic collection and analysis 
of field data resulted in the identifica- 
tion of seven geologic and five mining 
variables causally related to roof fail- 
ure. The five mining variables and four 
of the geologic variables were either in- 
appropriate for graphic representation or 
insufficiently detailed for delineation 
on a hazard zone map. Only 3 of the sig- 
nificant geologic variables — overburden 
thickness, roof lithology, and vertical 
distance to the rider coal — were useful 
in final map preparation; these variables 
were divided into 12 categories. Each 
variable was weighted and compared in a 
risk-interaction matrix, and final values 
were categorized into high- , moderate- , 
and low-risk designations. 

A separate transparent overlay map on 
a scale of 1:62,500 was prepared for 
each category. Ten categories were based 
chiefly on the lithologic character of 
roof rock at various levels above the 
coalbed, one was based on structure, and 
one was based on overburden thickness. A 
final hazard zone map (fig. 7) was pro- 
duced by combining 4 of the 12 categories 
after the weighting factors for probabil- 
ity and severity of the parameters were 
adjusted. 



12 



"'"^liiiii 




1 


L 


'1 


iff 


in 

liilii! 


1 


-. - 


i!i!!!i 


j 






!i 



'"I! 




4 



MM 



Niiuiii 





111 



! I 



i»i!! 






iii 



Up 



FIGURE 7. - Composite hazard zone map (33). Darkest areas indicate zones of highest risk. (The 
search radius for each data point (drill hole) was restricted to 1 mi.) 



Comment 

The overlay maps illustrating the 12 
geologic variables and the final hazard 
zone map representing an integration of 
several of these variables were conceived 
as a set of working maps that could be 
revised as additional detail is acquired 
through core drilling, mine development, 
or geologic studies. To facilitate the 
preparation and revision of such maps, 
the contractor prepared an explanatory 
text that describes in detail a computer 
program for data analysis and map print- 
out. This test also includes instruc- 
tions for inputting additional drill log 
data into the system for revision of the 
hazard maps. However, not all geologic 
variables can be programmed for computer 
analysis; some hazard risk zones must be 



developed manually through the use of 
overlays and geologic judgment. The suc- 
cess rate in predicting roof failure us- 
ing the method developed in this study is 
undetermined and will require consider- 
able site-specific experimentation to 
resolve. 

ENGINEERING STUDY OF STRUCTURAL GEOLOGIC 
FEATURES OF HERRIN (NO. 6) COAL AND 
ASSOCIATED ROCK IN ILLINOIS, 
CONTRACT H0242017 U9) 

This investigation summarized all ex- 
isting knowledge of the influence of the 
geologic fabric of roof strata on roof 
stability in mines in the Herrin (No. 6) 
Coalbed in Illinois. Conducted over a 
4-1/2-yr period, the investigation in- 
cluded underground work in operating 



13 



mines and a systematic compilation of 
existing information on file with the 
Illinois State Geological Survey. The 
geology of the Herrin roof rock was de- 
scribed in detail, and numerous struc- 
tural and stratlgraphic features were 
identified and related to roof 
conditions. 

Significant Findings 

The roof of the Herrin Coalbed consists 
of two major rock types: gray shale and 
black shale which in places includes 
a limestone bed. About 90 pet of the 
coalbed is overlain by the black shale, 
but the distribution patterns of the two 
rock types are intricate, irregular, and 
patchy. The position and thickness of 
the limestone member of the black shale 
is also variable. Sandstone is occasion- 
ally encountered in the roof. 

Commonly, mines in the Herrin Coalbed 
that have the most severe roof support 
problems have a gray shale roof. The 
problems are caused by roof rolls, a pro- 
trusion of the shale material into the 
upper portion of the coalbed (fig. 8). 
The rolls are usually accompanied by nu- 
merous slickensides or shear planes and 
thin coal stringers. The origin of these 
rolls is attributed to differential com- 
paction and deformation of the sediments 
while they were still in a plastic water- 
laden condition. 

Roof of the black-shale type is gener- 
ally stable where the shale is very thick 
or where the limestone member is at least 
2 ft thick. 

Roof consisting of laminated sandstone 
beds is difficult to support because the 
bedding surfaces are coated with plant 
material or mica which allows individual 
layers to separate and sag or fall. 

Roof support problems were occasion- 
ally encountered where claystone dikes 
with highly slickensided boundaries in- 
terrupted the continuity of the shale 
roof (fig. 9). The dikes were most com- 
mon where the limestone member of the 
black-shale group formed the immediate 



roof. These claystone dikes (or clay 
veins, in miners' terms) are irregularly 
shaped bodies of claystone that partially 
or completely transect the coalbed and 
extend into the roof. The claystone 
softens after prolonged exposure to hu- 
mid mine air, adding to the support prob- 
lem; and the slickensided dike is so 
poorly bonded to adjacent roof strata 
that the rock begins to fall when the 
supporting coal is removed — oftentimes 
before artificial support of any kind can 
be established. 

Joints occurring in mine roof were cor- 
related with minor slabbing of immediate 
roof but were not a cause of major roof 
falls. 

The individual geologic features or 
conditions described above all contri- 
buted to roof failure, but most of them 
were too small and too variable to be 
recognized or predicted entirely by means 
of normally spaced exploratory drill 
holes. Even major faults were difficult 
to recognize in widely spaced drilling. 
However, core drilling provided a general 
indication of the roof character and var- 
iability likely to be encountered during 
mining. 

Comment 

This study clearly showed that innumer- 
able and highly variable small-scale geo- 
logic features cause the major roof sup- 
port problems for mines in the Herrin 
Coalbed. Roof rock types, as well as in- 
dividual geologic features, are highly 
irregular in occurrence. Because of 
this, normally spaced core drilling alone 
can provide only the most general and 
imprecise information; usually the infor- 
mation is not adequate to delineate po- 
tentially hazardous zones. It is there- 
fore particularly important to use close 
spacing during exploratory core drilling 
and to revise assessments or predictions 
of roof conditions as soon as underground 
data begin to emerge from development 
work. An underground mapping program to 
permanently record easily recognized roof 
structures will provide the necessary 
data for revised predictions. 



14 




Splayed coal stringers 



Front coal "rider" over toe 



Body (showing almost concentric bedding) 



Normal fault (extensional 
and compactional) 



Postsedimentary "flow" of 
clastic material (mass movement) 



Toe with recumbent 
soft sediment folds 



Tail with small tail coal "rider" 
d extension fault 




FIGURE 8. - Roof rolls in the Herrin Coalbed (19). Top: Illustration shows coal stringers 
that have been split off from coalbed, folded Joes of the rolls, and low-angle normal faults 
that steepen downward and dissipate into the coal. Bottom: Typical soft-sediment protrusion 
of sandy material into surrounding coal, shales, and siltstones. 



Roof control plans and entry projec- 
tions should be flexible enough to pro- 
vide wide variability, because local 



geologic conditions that affect roof sta- 
bility can only be projected for short 
distances with any precision. 



REVIEW OF CONTRACT RESEARCH ON THE PHYSICAL PROPERTIES OF COAL MINE ROOF ROCK 



FAILURE OF ROOFS IN COAL MINES, 
CONTRACT H0232057 (2) 

This investigation was conducted over 
a period of 4 yr and chiefly involved 
three mines in the Herrin (No. 6) and 



Harrisburg (No. 5) Coalbeds in southern 
Illinois. The contract report, however, 
includes some findings of an earlier 
study of the deterioration of shales in 
coal mine roof (3). The objective of 
this investigation was to determine the 



15 



effects of exposure to the humid mine air 
on the coal mine roof. The investigation 
also included underground studies of the 
geometry and location of roof falls, 
long-range continuous monitoring of mine 
air humidity, and measurement of roof de- 
flection. Laboratory work was extensive 
and consisted of measuring the moisture 
penetration and absorption of roof shale 
and the resulting strain, temperature ef- 
fects, and physical properties of roof 
rock. 

Significant Findings 

Field Studies 

Monitoring at two operating mines 
showed that as air is taken into a mine 
it undergoes a rapid tempering; that 
is, the fluctuations in temperature and 
humidity are reduced by interaction 
with the underground environment. The 



distance inby at which the air becomes 
tempered and stable to nearly constant 
humidity and temperature is chiefly a 
function of the ventilation fan capacity 
for air flow, although the differential 
between surface and underground environ- 
ments is also a factor. 

Before surface air equalizes to under- 
ground conditions, there is an inter- 
change of moisture between the air and 
mine rock. During humid summer seasons, 
the surface air is cooled, and moisture 
condenses on mine rock surfaces. In win- 
ter, the air is drier and absorbs mois- 
ture in the mine. The monitoring studies 
showed that when humidity levels changed, 
the moisture interaction with the mine 
rock was most rapid during the first 6 
days after the change, and neither tem- 
perature nor barometric pressure had any 
significant effect on the moisture ab- 
sorption capacity of shale roof. 




FIGURE 9. - Claystone dike or clay vein transecting coalbed and shale roof. 



16 



While roof shale will absorb or release 
water depending on the moisture level in 
the air, the net effect on mine roof was 
a more or less continuous downward de- 
flection throughout all seasons despite 
short-term fluctuations in moisture, but 
some upward cycling of roof was observed. 
Measurements indicated that roof sag 
generally was greater in the spring and 
summer than in the fall and winter be- 
cause of greater humidity. Also, the 
more constant the humidity level was on a 
seasonal basis, the greater stability was 
in terms of roof sag. 

This study produced some indirect evi- 
dence that pointed to moisture penetra- 
tion through roof-bolt holes as the cause 
of high swelling pressures in the shale, 
which resulted in the loss of roof-bolt 
anchorage and failure of roof. 



summer. Laboratory tests confirmed the 
reduction in strength of roof shale with 
increased moisture. This evidence sug- 
gests that roof support problems in mines 
with moisture-sensitive roof rock such as 
clay shale and claystone might be reduced 
by holding humidity levels to a minimum, 
especially during spring and summer. 
This could be accomplished by diverting 
intake air through tempering entries. 

No reliable quantitative tests for 
moisture sensitivity of roof rock were 
developed. Simple qualitative exposure 
or immersion tests probably remain the 
most practical method for assessing the 
slaking potential of roof shales. 

EFFECTS OF TEMPERATURE AND HUMIDITY 

VARIATIONS ON STABILITY OF COAL MINE 

ROOF ROCKS, CONTRACT H0122111 (16) 



Laboratory Studies 

None of the numerous physical tests on 
mine roof shale samples proved satisfac- 
tory for quantitative evaluation of the 
shale for roof stability, although all 
tests demonstrated a reduction in shale 
strength with increased moisture. Slak- 
ing tests were inconclusive. No individ- 
ual physical property or diagnostic test 
of shale roof rock yielded reliable esti- 
mates of roof behavior, as shown by com- 
parison of these estimates with the field 
monitoring data. However, laboratory 
studies did reveal that water penetration 
of roof shale is 50 pet greater along 
bedding than it is perpendicular to 
bedding and that penetration is rapid up 
to 1/4 in deep, then greatly decreases. 
Moisture absorption in shale samples re- 
sulted in swelling pressures of up to 
4,200 psi, according to calculations 
based on laboratory data. 

Comment 



The objectives of this contract were 
similar to those of the preceding one 
(contact H0232057): to determine the re- 
sponse of shale roof to variations in 
temperature and relative humidity, to de- 
velop a reliable moisture sensitivity 
test for roof rock samples, and to apply 
the findings to mine design. The inves- 
tigation was conducted at four mines in 
the Warrior coalfield of central Alabama 
operating in the Mary Lee, America, and 
Pratt Coalbeds. Roof rocks consisted 
principally of carbonaceous clay shales 
with some argillaceous sandstones. The 
underground studies consisted of periodic 
roof-strain measurements made with a me- 
chanical device and continuous recording 
of temperature and humidity levels under- 
ground and on the surface. The labora- 
tory studies consisted of strain measure- 
ments under controlled humidity and 
temperature on samples of roof rock using 
a wire-resistance strain-gauge technique. 
The physical and mineralogical properties 
of rock samples were determined. 



This study described the tempering of 
humid intake air, the Interaction of the 
air with shale roof, and the resulting 
roof sag. The roof sag was more or less 
continuous but greater in the spring and 
summer than in fall and winter because 
of increased humidity during spring and 



Significant Findings 

Field Studies 

Measurements of mine air humidity lev- 
els and roof strain parallel to bedding 



17 



yielded widely scattered data and poor 
statistical correlation. Roof-strain 
values were much lower than those devel- 
oped in roof rock samples tested in labo- 
ratory environmental control chambers. 
The average measured roof strain under- 
ground was only 371 pin/in, or 16 pet of 
the average strain developed in the labo- 
ratory under similar humidity and temper- 
ature conditions, and probably was low 
due to the in situ confinement. 

Moisture balance calculations showed 
that the mines tested absorb moisture 
during the summer and expel it during 
winter. This indicates that air temper- 
ing is needed mostly during summer months 
when moisture levels in the atmosphere 
are highest. Daily temperature varia- 
tions underground ranged from 2° at 11° F 
at distances greater than 1,000 ft inby 
the portal and should directly cause only 
insignificant amounts of roof strain. 
Evidence of roof spalling was abundant in 
air-intake entries but was virtually un- 
detectable in returns. 

Laboratory Studies 

The wire-resistance strain-gauge tech- 
nique used in these studies provided 
accurate results for measurements of 
moisture-induced strain in roof rock sam- 
ples performed in an environmental con- 
trol chamber. The strain in samples of 
roof from Alabama coal mines ranged from 
200 to 6,000 yin/in under simulated mois- 
ture and temperature conditions. The 
strain developed in these rock samples by 
normal temperature variations only was 
far less than that noted for humidity 
variations and should not contribute to 
roof problems. The roof rock samples re- 
acted gradually to humidity changes over 
a period of 7 to 10 days before the de- 
veloped strain stabilized. This slow ad- 
justment to humidity changes seems to 
rule out daily humidity cycles as a fac- 
tor contributing to roof problems. 



Physical property tests on roof shale 
samples (slake durability, swelling in- 
dex, and Shore hardness) did not corre- 
late well with moisture sensitivity. No 
correlations were observed between the 
relative amounts of aluminum, silicon, 
iron, calcium, or magnesium; nor were the 
results of X-ray diffraction, mineralogi- 
cal content, or petrographic description 
of any value in determining the sensitiv- 
ity of roof shale to moisture. No mont- 
morillonite or other interlayered clay 
minerals that swell on wetting were 
detected. 

Comment 

This study indicated that samples of 
roof rock, such as might be obtained 
from exploratory drill holes , could be 
tested in the laboratory for sensitivity 
to moisture using the wire-resistance 
strain-gauge technique in an environmen- 
tal control chamber. While this method 
of testing would be impractical for most 
mining companies , selected samples could 
be tested in a commercial laboratory to 
confirm the results of far simpler expo- 
sure or immersion tests that could be 
performed at a field office. The poor 
correlation of laboratory and underground 
data, however, suggests that the utility 
of such laboratory tests is seriously 
limited. 

As in the studies done under contract 
H0232057, evidence was presented that 
suggests the possibility of using air- 
conditioning chambers during the summer 
months to reduce humidity entering the 
mine and water sprays in winter to in- 
crease humidity. These opposing systems 
would serve to reduce the wide fluctua- 
tions in the moisture content of intake 
air. 

CORRELATION OF MINE ROOF FAILURE WITH 
TIME ELAPSE BEFORE SUPPORT 
INSTALLATION, CONTRACT H01 11413 (5) 



The data on moisture-induced strain ob- 
tained in the laboratory did not compare 
well with the data obtained underground, 
although a high correlation coefficient 
was obtained for the laboratory data. 



The purpose of this investigation was 
to determine if a significant relation- 
ship exists between roof areas that are 
left unsupported for several hours or 
days and roof areas that eventually fail. 



18 



It is included in this review because 
geologic factors and their bearing on 
roof support also were studied. 

The research was conducted at three un- 
derground mines operating in the Mary Lee 
Coalbed in the Warrior Basin of central 
Alabama. Depth of overburden ranged from 
200 to 700 ft. Emphasis was on a system- 
atic study of mining and support instal- 
lation sequences, roof deflection, and 
roof rock, properties. A finite-element 
model of a mine entry was constructed us- 
ing assumed stress conditions and physi- 
cal properties adapted from a laboratory 
test conducted on samples of roof rock. 

Significant Findings 

The results of time-lapse studies at 
three study sites were not consistent. 
At one site, there was no significant 
difference in roof stability between 
areas immediately supported by bolting 
and areas left unsupported for an ex- 
tended period. At the two other sites, 
the areas of roof that eventually failed 
were left unsupported for two to three 
times the normal interval. Statistical 
analysis of roof fall data, however, 
showed a significant correlation at all 
three sites between roof fall area and 
the roof -bolt pattern, where the bolt 
pattern was defined as the bolt length 
times the bolt spacing squared. Other 
variables such as mining depth and entry 
geometry were of minor influence on roof 
stability. 

Visual examination of the roof and roof 
falls at each study site revealed that 
almost all of the falls were closely as- 
sociated with faults, joints, roof rolls, 
a contrast in roof lithology such as 
crossbedded sandstone lenses in predomi- 
nantly shale roof, or other sedimentary 
structures. 

Roof -deflection data, as measured by 
numerous rod-type single-position bore- 
hole extensometers installed in groups of 
three at depths of 5, 10, and 20 ft into 
the roof, was inconclusive because of 
suspected, but uncorrected, temperature 
effects. Extensometers were located at 



distances of 1 to 150 ft from the face 
and detected maximum movement of from 
0.006 to 0.089 in, most of which occurred 
in the first foot or two of roof. 

Comment 



In this study, the geologic conditions 
at each underground site far overshadowed 
the effects of time lapse between excava- 
tion and permanent support. While roof 
failure frequently occurred where roof 
was left unsupported for extended peri- 
ods, the correlation was not good; it 
seems more important to recognize chang- 
ing geologic conditions as a key requi- 
site for effective roof support. This 
would entail a considerable ability to 
assess roof structure and a flexibility 
in supplementary roof support methods. 
Nonetheless, the study presented some 
evidence that time lapse is a significant 
factor, particularly where mining is car- 
ried out under structurally weak roof. 

CAUSE AND PREVENTION OF FAILURE OF 
FRESHLY EXPOSED SHALE AND SHALE 
MATERIALS IN MINE OPENINGS, 
CONTRACT G01 11809 (_4) 

The objective of this study was to 
determine the mechanism by which coal 
mine roof rocks interact with mine atmos- 
pheres and weaken. The scope of the work 
was limited to some underground observa- 
tions and chiefly laboratory tests on 
samples of shale roof rock to determine 
how the rock changed after exposure to 
controlled humidity levels. Samples for 
^testing were collected from coal mines 
near Price, UT; western Washington; Weld 
County, CO; and Raton, NM. Roof condi- 
tions generally were Dad at the sampling 
sites, although the cause of the bad roof 
was unknown. The laboratory tests in- 
cluded mineralogical determinations , 
Shore hardness, slake durability, fabric 
analysis, sonic velocity, and electron- 
beam microanalysis. 

Significant Findings 

Shore hardness decreased and speci- 
men weight increased up to 1.7 pet af- 
ter shale roof samples were exposed to 



19 



100-pct relative humidity. Constant 
weight was reached in 4 to 7 days. Any 
abrupt increase from a low to a high hu- 
midity (of nearly 100 pet) caused many 
specimens to break, although gradual in- 
creases avoided breakage. Shore hard- 
ness , considered by some to be a rough 
indication of rock competence, increased 
with silica content of shale and de- 
creased with clay content. No evidence 
of sample swelling was detected by X-ray 
diffraction. However, there was a lack 
of correlation between all the laboratory 
tests performed and the observed behavior 
of large pieces of shale both within the 
mine and in the laboratory. Montmoril- 
lonites were seldom found in the roof 
samples studied, and quartz was the only 
nonclay mineral present in major amounts. 
Except for montmorillonite, the same min- 
erals occurred in all samples of roof, 
but the relative amounts varied widely 
even in different portions of the same 
specimen. 

Comment 

This project characterized the diffi- 
culties and failures that have been com- 
mon in attempts to perfect a reliable 
physical property test that alone shows a 
useful correlation with the weakening ef- 
fects on roof rock exposed to humid mine 
atmosphere. The problem is complex be- 
cause the textural and mineralogic attri- 
butes of roof shale are highly variable, 
while microscopic features such as small 
fractures, slickensides , laminations, and 
internal structures predominate as fac- 
tors in overall strength and are diffi- 
cult to distinguish even in laboratory 
specimens . 

Finding a practical but quantitative 
and measurable parameter that is a diag- 
nostic of roof shale stability in humid 
mine air remains a challenge. 

CONTROL OF SHALE ROOF DETERIORATION WITH 
AIR TEMPERING, CONTRACT JO 188028 (6) 

The objective of this contract was to 
evaluate the effectiveness and feasibil- 
ity of using air tempering (AT) entries 



in coal mines to reduce or stabilize hu- 
midity levels in mine air and thereby 
control roof slaking. 

The long-term effects of mine air hu- 
midity on shale roof have not been fully 
assessed, although roof disintegration is 
severe in many intake entries and near 
shaft bottoms, and laboratory tests have 
confirmed that moisture invariably weak- 
ens samples of shale roof. The use 
of multiple entries or rooms as air- 
conditioning chambers to control roof 
disintegration has been attempted with 
varying degrees of success , but documen- 
tation has been poor. 

Under this contract , all available lit- 
erature relating to AT was compiled and 
an annotated bibliography was prepared. 
This was followed by long-term on-site 
monitoring of a mine where the effective- 
ness of air-conditioning chambers was 
fully documented. The results of this 
investigation were used to develop design 
criteria for the use of AT chambers. 
Also, a moisture sensitivity test was de- 
veloped for use on exploratory drill-core 
samples of roof shale. 

The mine air and roof conditions in the 
AT and inby entries of a mine near Wheel- 
ing, WV, were monitored for 12 months. 
The air temperature, air humidity, baro- 
metric pressure, and flow rates were mea- 
sured almost continuously, while roof 
disintegration in the AT entries was as- 
sessed periodically through convergence 
measurements, the collection of fallen 
material, Schmidt Hammer tests, photo- 
graphic logging, changes in roof height, 
and detailed mapping. 

Significant Findings 

Field Studies 

The monitoring of mine air clearly 
showed seasonal effects. During winter, 
temperatures are low and consequently 
absolute humidity is low, so the roof 
shale is deprived of moisture. During 
the summer, absolute humidity and temper- 
ature rise, so the roof shale absorbs 



20 



moisture. Thus, shale roof is subject to 
extreme wet and dry cycles which are 
greater near the shaft than farther inby 
and beyond the AT entries. Temperature 
equilibrium was largely complete within 
5 min residence time, while humidity re- 
quired about 30 min to stabilize. Veloc- 
ity within the AT entries should be less 
than 300 ft/min. In midsummer, air tem- 
perature drops radically as air passes 
into the AT entries, condensation occurs 
on the mine roof and walls , and the at- 
mospheric moisture content drops rapidly 
until about 20 min air residence time. 
In midwinter, colder temperatures reduce 
the absolute humidity of intake air to 
low levels; however, there is little 
exchange of moisture between rock and 
air, due to dryness of the rock, until 
after warming takes place and some mois- 
ture is re-absorbed by the air. Air tem- 
pering thus has the effect of evening out 
both temperature and humidity levels be- 
fore the air is diverted into the main 
entries. 

During midsummer, the AT entries were 
foggy and roof surfaces were wet. Deter- 
ioration of the roof was marked by almost 
continuous falls of small pieces of roof 
rock. Deterioration was most severe near 
the inlet, where the air was warm, and 
near clay veins. Near the outlet to the 
main entries, the air was cooler and 
clearer, the roof was dry, and the roof 
fragments fell much less frequently. In 
winter, the AT entries were cool and dry, 
and roof disintegration stopped. Little 
deterioration of the main entries oc- 
curred throughout the year. The zones 
and times of maximum roof sag correlated 
with the changing positions of mine air 
equilibrium zones. Some initial disinte- 
gration of roof was attributed to remnant 
fugitive moisture from the developmental 
mining cycle. Roof consisting of unlami- 
nated, slickensided claystone disinte- 
grated rapidly on exposure, while gray 
and black laminated shale showed no slak- 
ing tendency. 

Laboratory Studies 

Samples of roof rock were subjected 
to tests for static slaking, natural 



moisture content, null-point humidity (at 
which the rock will neither take up nor 
give up moisture), and shale expansion at 
various humidity levels. These tests 
were conducted to determine if the re- 
sults would be indicative of roof shale 
deterioration upon exposure to variable 
humidity levels. Only shale expansion 
gave a reliable indication of potential 
disintegration, but the method used is 
somewhat complicated and costly for rou- 
tine use. 

Cost Benefit 

The cost effectiveness of using AT en- 
tries was estimated on the basis of sev- 
eral assumptions because reliable val- 
ues for the various factors involved 
were unavailable. The AT entries used 
at the mine studied were shown to be 
cost effective, due to a reduced need 
for maintenance in the main entries. 
Although AT entries have a high initial 
cost and deteriorate with time (account- 
ing for the term "sacrificial entries"), 
the cost is more than offset by lower 
operating costs. In addition, the hazard 
of roof spalling in the main entries is 
reduced. 

Comment 



This investigation constitutes the 
first well-documented assessment of AT 
entries and, as such, may not be repre- 
sentative of other mining districts where 
different climates , conditions , and roof 
rock prevail. However, for the upper 
'Ohio River Valley, this study demon- 
strated that wide ranges in humidity lev- 
els can be controlled. While the AT 
entries were subject to severe roof dis- 
integration, the roof of the main entries 
was not subject to wide fluctuations in 
humidity and remained stable, requiring 
little if any maintenance. The study in- 
dicated that the moisture sensitivity of 
samples of roof rock can be estimated by 
using a shale expansion test performed on 
exploratory drill-core samples prior to 
mining. This, along with some of the de- 
sign criteria developed for AT entries, 
should provide mine planning engineers 
with better guidelines for controlling 



mine air humidity than were previously 
available. It was concluded that the 
life of AT entries is impossible to 
predict but can be extended by the 



21 



installation of wire mesh on the roof or 
some degree of maintenance such as clean- 
up, re-posting, and/or re-bolting. 



CONCLUSIONS REGARDING GEOLOGIC STRUCTURE AND LITHOLOGIC CHARACTER OF MINE ROOF 



In the studies conducted under three 
contracts H0133018, H0230028, and 
H0242017, in which the geologic character 
of mine roof and its relation to roof 
stability were assessed, two structures 
were found to predominate over all other 
features in the areas studied as factors 
that directly contribute to a large por- 
tion of roof failure. These are roof 
rolls and slickensides. 

ROOF ROLLS 

This term includes any abrupt downward 
protrusion of sandstone or shale roof 
rock into the top of the coalbed or into 
the shale and coal rash immediately above 
the coalbed. A common form of roof roll 
is shown in figures 1 and 4. Roof rolls 
may consist of individual troughlike 
sandstone- or shale-filled channels and 
scours or the convex undulations at the 
base of a thick sandstone member of roof 
rock. Slickensides invariably occur 
around the bases and flanks of the rolls, 
which along with the rolls interrupt the 
continuity and beam structure of the roof 
strata. In addition to the slickensides, 
the effects of differential compaction 
marginal to roof rolls include fractur- 
ing, small-scale bending and folding, and 
minor faulting. Rolls consisting of per- 
meable sandstone commonly contain ground 
water which is released into the mine en- 
tries, thereby contributing to problems 
of mine haulage and roof deterioration. 
Roof falls are most likely to occur adja- 
cent to the rolls and beneath the base, 
where slickensides are most prevalent. 
The hazardous nature of paleochannels 
(roof rolls) has been described by Moebs 
and Ellenberger (22). 

Rolls can be identified and mapped un- 
derground and sometimes projected from 
400 to 2,000 ft into unmined coal if the 



linear trend is very pronounced, although 
many rolls are discontinuous. 

Only a general indication or probabil- 
ity of roll occurrence can be deduced 
from drill-core data. The presence in 
core of conglomerate, irregular coal 
streaks, and distorted bedding might in- 
dicate the proximity of a roll. Varia- 
tions in the interval between the coalbed 
and the base of a thick sandstone suggest 
an undulating contact that might be trou- 
blesome if it is within 15 ft of the 
roof. 

SLICKENSIDES 

Slickensides are particularly common 
around the margins of roof rolls but oc- 
cur throughout most fine-grained rocks 
such as claystones (fig. 3), also known 
as clod rock or seat earth, and clay 
shales. Slickensides constitute one of 
the most common causes of roof falls of 
all sizes because they interrupt the 
continuity of roof strata, forming wedge- 
shaped segments of rock that are diffi- 
cult to support. They can be identified 
readily in drill core, but any prediction 
as to their distribution and density is 
best based on the distribution of their 
host rock, because a slickenside count 
from drill core represents an exceedingly 
small and probably unreliable sampling. 

The preceding relationships were estab- 
lished through the contract studies con- 
ducted in eastern Kentucky, southern West 
Virginia, and southern Illinois. Similar 
relationships were observed in many mines 
in the Pittsburgh Coalbed of southwestern 
Pennsylvania, indicating the widespread 
occurrence of these types of roof prob- 
lems. Roof rolls at one mine in south- 
western Pennsylvania are described in de- 
tail in a Bureau report by Moebs (21), 



22 



while a study of slickensides at a mine 
in the northern West Virginia panhandle 
is discussed by Ellenberger in another 
report (11) . 

Supplementary roof support for rolls 
and slickensides customarily consists of 
bolting, using blocks and metal straps, 
with an occasional post and crossbar or 
rail. Angle bolting may offer some ad- 
vantage but rarely is used. 

In addition to roof rolls and slicken- 
sides, three features of lesser impor- 
tance in roof instability emerged from 
these contract studies: interlaminated 
shale-sandstone-coal, joints, and clay 
dikes (or clay veins). These three fea- 
tures are discussed below. 

INTERLAMINATED SHALE-SANDSTONE-COAL 

The combination of poorly bonded, thin- 
ly bedded, and often rippled strata with 
mica-rich layers is nearly always diffi- 
cult to support and prone to separate and 
fail progressively layer by layer. Ac- 
cording to Moebs (22-23) , this rock unit 
can best be described as a crevasse 
splay, and it occurs most commonly in the 
lower part of a sandstone. It is known 
to miners as trashy or rashy sandstone, 
or stackrock if it is predominately sand- 
stone. Examples of shale-sandstone-coal 
are shown in figures 4 and 5. Interlami- 
nated strata can be readily identified in 
drill core. The lateral distribution of 
this type of roof is erratic, but can be 
approximately outlined; and the thickness 
can be represented by an isopach map. 
Experience indicated that in either lami- 
nated or soft roof, resin bolting is more 
effective than mechanical bolting. 

JOINTS 



and then only under shallow cover, essen- 
tially in the zone of weathering under 
stream valleys and near the outcrop line. 

In areas of the northern Appalachian 
region where overburden is less than 600 
ft, over 90 pet of severe roof instabil- 
ity, often called "snap top," occurs be- 
neath narrow steep-walled valleys , ac- 
cording to a preliminary Bureau survey of 
operating mines. Underground evidence 
strongly indicates lateral stresses due 
to topographic effects as the cause, 
rather than jointing, which was first 
suspected. Roof instability caused by 
high lateral compressive stresses is usu- 
ally characterized by the shears or "cut- 
ters" that develop at the intersection of 
roof and rib (fig. 10). Shears can some- 
times be controlled by angle bolting. 
Under severe stress conditions, roof 
bolting alone will seldom support the 
roof, and posts with crossbar or cribbing 
must be used, or reorientation of work- 
ings may help. 

Even though jointing was not considered 
a major cause of roof failure in the 
mines studied with respect to the geo- 
logic character of mine roof, the authors 
recognize that joints are sometimes sig- 
nificant contributing factors to roof 
instability. For example, the final re- 
port for contract HOI 11413 repeatedly 
points out the relationship of joints to 
roof falls that were studied in that in- 
vestigation. Stateham (31) encountered a 
joint-controlled area of roof instability 
in a study in Colorado. Scheibner (28) 
mentions joints as a major contributor to 
roof falls in Utah. On the basis of very 
limited information, joint -related roof 
falls appear to be more prevalent in the 
Western United States than the Eastern 
United States. 



Care must be exercised in distinguish- 
ing between a slickenside — a usually 
curving, polished, and striated surface — 
and a joint — a plane surface (always ver- 
tical, or nearly so, in the bituminous 
coal region) that occurs in parallel sets 
of widely varying density. Only one con- 
tractor found jointing to be of even 
doubtful significance to roof conditions, 



CLAY DIKES 

Clay dikes, or clay veins, were identi- 
fied by only one contractor as constitut- 
ing an occasional roof support problem in 
the Illinois Basin. Available data for 
other areas were incomplete. While clay 
dikes occur in many coalbeds over a wide 
area of the northern Appalacian coal 



23 




mmxmj 



Entry 




ffDrciw slate/%%e$ y 



Coalbed 



Underclay' 





i L 



Scale, ft 
FIGURE 10. • Shear- or "cutter"-type roof failure developing along rib line. 



region, they are particularly large and 
abundant, and a serious problem, in 
the Pittsburgh Coalbed in the upper Ohio 
River Valley. Here, as in the Her r in 
Coalbed in Illinois, clay dikes are most 
common where the immediate roof consists 
of a few feet of clays tone or clay shale 
overlain by limestone. Clay dikes also 
occur in the West, where they are more 
commonly called spars or rock spars. 
Dunrud ( 10 ) mentions their presence in 
United States Steel Corp.'s Somerset Mine 
and states that they caused the closure 
of another mine (the Cameo Mine). 

Clay dike widths range from a few 
inches to over 12 ft. The narrow dikes 
resemble clay-filled faults or joints, 
while the wide ones are broad and trough- 
like (figs. 2 and 9). Clay dikes are 



highly erratic in lateral extent but 
often exhibit a dendritic or desiccation 
pattern. 

Clay dikes are virtually unpredictable 
by core drilling alone, although their 
occurrence might be conjectured based 
upon existence of the proper host rock, a 
soft claystone overlain by limestone or 
calcareous shale. Otherwise, projections 
based on underground mapping offer the 
best prospects for predictions. For ex- 
ample, important intersections and en- 
tries can be located so as to avoid the 
projections of significant clay dike 
trends . 

The customary roof support consists 
of supplementary bolting using blocks, 
planks, or metal straps. 



24 



CORE MANUAL AND HAZARD MAP 

The potential value of contract 
J0188115, which provided for publica- 
tion of a guidebook for drill-core iden- 
tification and classification, is self- 
evident and has been discussed. The 
degree to which the book is used will be 
a large factor in the success of future 
premining investigations that are based 
on descriptive drill-core data. 

Similarly, the potential value of con- 
tract J0177038 rests with the application 
of the hazard zone base map concept 
and constant revision of the resulting 
maps as new information emerges from 



geologic studies or underground develop- 
ment. While the original base maps can 
be prepared through sophisticated com- 
puter analysis and plotting of drill-hole 
data, follow-up manual modifications in 
specific areas are essential to attain 
the maximum precision, and geologic judg- 
ments of the highest order are required. 

The concept of a hazard potential map 
is not altogether new. One of the first 
examples devised was illustrated in Bu- 
reau of Mines Technical Progress Report 
70 in 1973 (24). However, because of the 
subtle geologic complexities involved, it 
has not been widely adopted. 



CONCLUSIONS REGARDING PHYSICAL PROPERTIES OF COAL MINE ROOF ROCK 



ROOF DISINTEGRATION AND HUMIDITY 

Of the five contract studies relating 
to the physical properties of roof rock, 
four attempted to resolve the problem of 
roof disintegration after exposure to hu- 
mid mine air. Three of the studies (con- 
tracts H0232057, H0122111, and G0111809) 
together showed the following: 

1. The weakening effects of moisture 
on shale roof were confirmed. 

2. Both humidity and roof sag are 
greater in the spring and summer than in 
the fall and winter, but sag is more or 
less continuous throughout the year. 
(However, these findings are in contrast 
with the results of Bureau measurements 
which show virtually no sag in test areas 
for up to 2 yr. ) 

3. The more constant the humidity sea- 
sonally, the less the sag. 

4. Temperature changes are insignifi- 
cant in roof stability. 

5. Only seasonal and not daily humid- 
ity variations have a significant effect 
on roof stability. 

6. The moisture sensitivity of rock 
can be detected in the laboratory by mea- 
suring developed strain or changes in 



Shore hardness. However, a simple and 
reliable test for use on drill-core sam- 
ples to predict the weakening effect of 
high humidity was not developed, largely 
because macroscopic features predominate 
over the effects of moisture. When used 
together, however, the laboratory tests 
and macroscopic features can be useful 
indicators of rock stability. 

These findings support the conclusion 
reached in other reports (30-31) that 
humidity influences roof fall occurence 
rates. However, the severity, size, and 
distribution of roof fall occurrences re- 
main undetermined, as does a method for 
reducing them. 

A fourth study (contract J0188228) was 
directed at evaluating one of the few, 
yet controversial, methods for controll- 
ing humidity in a mine, the use of air 
tempering chambers. Few options are open 
to the operator for limiting humidity in 
mine air. One method that has been at- 
tempted in past years, but was poorly 
documented, is to use so-called temper- 
ing, sacrificial, or air-conditioning 
chambers in which air is passed through 
several long parallel entries at low ve- 
locity to increase residence time. In 
the summer, moisture is condensed and ab- 
sorbed on rock surfaces in these cham- 
bers, and therefore less moisture reaches 
the inby workings. During fall and 



25 



winter, dry incoming air recovers some 
moisture. Thus, the range in humidity is 
narrowed. The roof over these entries 
may disintegrate severely after a few 
years of use, as is common near the bot- 
tom of air-intake shafts. 

Water sprays have also been used during 
the winter to maintain a moderate level 
of humidity in the otherwise dry air, 
thus preventing wide fluctuations from 
season to season; however, water handling 
and ice formation can be troublesome. 

Sealants, to exclude air from roof 
shale, are used widely in entries near 
shafts and portals, but are costly and 
leave inby entries unprotected. 

Moisture effects are subtle and diffi- 
cult to assess, but it was concluded from 
monitoring at the Valley Camp Mine near 
Wheeling, WV, that air tempering entries 
are effective in controlling humidity and 
roof deterioration. 

TIME LAPSE BEFORE ROOF SUPPORT 
INSTALLATION 

The studies conducted under contract 
HOI 11413 indicated that while the time 
lapse between roof exposure and perma- 
nent support may be a factor in long-term 
roof stability, particularly where roof 
conditions are bad, the relation is 



difficult to establish. This is because 
the geologic character of the roof strata 
is a much greater and highly variable 
factor that probably contributes to the 
time-lapse relationship. A fuller as- 
sessment of the influence of roof geology 
on roof movement will be required before 
the effects of a time lapse prior to sup- 
port can be reliably determined (26-27, 
3J0. 

In similar studies conducted by the Bu- 
reau in Colorado (28, 30 , 32 ) it was con- 
cluded that once permanent support is 
achieved, the amount of time the roof was 
left does not appear to affect long-term 
roof stability in the mine investigated. 
These studies were conducted mainly in 
relatively strong roof rock. Roof sag 
was monitored in a mine section where 
roof bolting was delayed for up to 88 h 
after exposure. The rate of sag was high 
immediately after mining and before bolt- 
ing, but fell to a low value and stabi- 
lized after bolts were installed. Also, 
roof fall occurrence was compared to time 
lapse, but no positive correlation was 
found. The roof in the mine studied was 
shale to sandy shale overlain by sand- 
stone and was intensely slickensided 
(31) . A total of five study areas were 
instrumented. One area was influenced by 
jointing. Another area was influenced by 
a roof roll and also by the swelling ef- 
fects of montmorillonite when wet. 



REFERENCES 



1. Adler, L. , and M. C. Sun. Ground 
Control in Bedded Formations. VA Poly- 
tech. Inst. Blacksburg, VA, Res. Div. 
Bull. 28, Dec. 1968, 226 pp. 

2. Aughenbaugh, N. B. , and R. F. Bru- 
zewski. Humidity Effects on Coal Mine 
Roof Stability (contract H0232057, Univ. 
MO, Rolla, MO). BuMines OFR 5-78, 1976, 
164 pp.; NTIS PB 276 484/AS. 

3. . Investigation of the Fail- 
ure of Roofs in Coal Mines (contract 
HOI 11462, Univ. MO, Rolla, MO). BuMines 
OFR 55-75, 1973, 135 pp.; NTIS PB 243 
375/ AS. 



4. Bobeck, G. E. , and D. F. Clifton. 
Cause and Prevention of Failure of Fresh- 
ly Exposed Shale and Shale Materials in 
Mine Openings (contract G01 11809, Univ. 
ID). BuMines OFR 31-74, 1973, 116 pp.; 
NTIS PB 232 891/AS. 

5. Cox, R. The Correlation of Mine 
Roof Failure With the Time Elapse Be- 
fore Support Installation Final Report, 
(contract HOI 11413, Univ. AL). BuMines 
OFR 11-77, 1974, 80 pp.; NTIS PB 
262 478/AS. 



26 



6. Cummings , R. A., M. M. Singh, S. E. 
Sharp, and A. W. Laurito. Control of 
Shale Roof Deterioration With Air Temper- 
ing (contract J0188028, Eng. Int., Inc.). 
Volume 1 : Field and Laboratory Investi- 
gations. BuMines OFR 41(l)-82, 1981, 164 
pp.; NTIS PB 82-199985; Volume 2: Anno- 
tated Bibliography. BuMines OFR 41(2)- 
82, 1981, 64 pp.; NTIS PB 82-199993. 

7. Dahl, H. D. , and R. C. Parsons. 
Ground Control Studies in the Humphrey 
No. 7 Mine, Christopher Coal Division, 
Consolidation Coal Co. Trans. Soc. Min. 
Eng. AIME, v. 252, June 1972, pp. 211- 
222. 

8. Diessel, C. F., and K. H. Moelle. 
The Application of Analysis of the Sedi- 
mentary Structural Features of a Coal 
Seam and Its Surrounding Strata to Oper- 
ations of Mining. Pres. at the 8th Com- 
monwealth Min. and Met. Congr. , Australia 
and New Zealand, Melbourne, Australia, 
Feb. 11, 1965. Office of the Congr. and 
the Australias. Inst, of Min. and Met. 
preprint 36, 22 pp. 

9. Donaldson, A. C. Some Appalachian 
Coals and Carbonates: Models of Ancient 
Shallow-Water Deposition. WV Geol. and 
Econ. Surv. , WV Univ., Morgantown, WV, 
Nov. 1969, 384 pp. 



13. Ferm,. J. C. , and G. C. Smith. 
Methods and Criteria for Producing a Pho- 
tographic Core Logging Manual for the 
Pittsburgh Basin. Final report on Bu- 
Mines contract J0188115 with Univ. SC, 
Jan. 1983, 93 pp.; available upon request 
from Noel N. Moebs, BuMines, Pittsburgh, 
PA. 

14. Greenwald, H. P., I. Harmann, 
E. R. Maize, and G. S. Rice. Studies of 
Roof Movement in Coal Mines. 1. Montour 
10 Mine of the Pittsburgh Coal Co. Bu- 
Mines RI 3355, 1937, 41 pp. 

15. Hartmann, I., and H. P. Greenwald. 
Effects of Changes in Moisture and Tem- 
perature on Mine Roof. 1. Report on 
Strata Overlying the Pittsburgh Coal Bed. 
BuMines RI 3588, 1941, 40 pp. 

16. Haynes, C. D. Effects of Tempera- 
ture and Humidity Variations on the Sta- 
bility of Coal Mine Roof Rocks (contract 
H0122111, Univ. AL). BuMines OFR 8-77, 
1975, 385 pp.; NTIS PB 262 516/AS. 

17. Holland, C. T. Structure of Mine 
Roof and Some of Its Effects On Roof Con- 
trol. Paper in Proceedings of the West 
Virginia Coal Mining Institute 41st An- 
nual Meeting, WV Coal Min. Inst. , Morgan- 
town, WV, 1948, pp. 85-107. 



10. Dunrud, C. R. Some Engineering 
Geologic Factors Controlling Coal Mine 
Subsidence in Utah and Colorado. U.S. 
Geol. Surv. Prof. Paper 969, 1976, 
39 pp. 

11. Ellenberger, J. L. Slickenside 
Occurrence in Coal Mine Roof of the Val- 
ley Camp No. 3 Mine Near Wheeling, W. 
Va. BuMines RI 8365, 1979, 17 pp. 

12. Ferm, J. C. , R. A. Melton, G. D. 
Cummins, D. Mat hew, L. L. McKenna, 
C. Muir, and G. E. Norris. A Study of 
Roof Falls in Underground Mines on the 
Pocahontas No. 3 Seam, Southern West Vir- 
ginia and Southwestern Virginia (contract 
H0230028, Univ. SC). BuMines OFR 36-80, 
1978, 92 pp.; NTIS PB 80-158983. 



18. Hylbert, D. K. Developing Geolog- 
ical Structural Criteria for Predict- 
ing Unstable Mine Roof Rocks (contract 
H0133108, Morehead State Univ.). BuMines 
OFR 9-78, 1977, 249 pp.; NTIS PB 276 
735/AS. 

19. Krausse, H. F. , H. H. Damberger, 
W. J. Nelson, S. R. Hunt, C. T. Ledvina, 
C. G. Treworgy, and W. A. White. Engi- 
neering Study of Structural Geologic Fea- 
tures of the Herrin (No. 6) Coal and 
Associated Rock in Illinois (contract 
H0242017, IL State Geol. Surv.) Volume 1: 
Summary Report. BuMines OFR 96(l)-80, 
1979, 67 pp.; NTIS PB 80-219454; Volume 
2: Detailed Report. BuMines OFR 96(2)- 
80, 1979, 218 pp.; NTIS PB 80-219462. 



27 



20. McCulloch, C. M. , W. P. Diamond, 
B. M. Bench, and M. Deul. Selected Geo- 
logic Factors Affecting Mining of the 
Pittsburgh Coalbed. BuMines RI 8093, 
1975, 72 pp. 

21. Moebs, N. N. Roof Rock Structures 
and Related Roof Support Problems in the 
Pittsburgh Coalbed of Southwesten Penn- 
sylvania. BuMines RI 8230, 1977, 30 pp. 

22. Moebs, N. N. , and J. L. Ellenber- 
ger. Geologic Structures in Coal Mine 
Roof. BuMines RI 8620, 1982, 16 pp. 

23. Moebs, N. N. , and J. C. Ferm. The 
Relation of Geology to Mine Roof Condi- 
tions in the Pocahontas No. 3 Coalbed. 
BuMines IC 8864, 1982, 8 pp. 

24. Overbey, W. K. , Jr. , C. A. Komar, 
and J. Pasini, III. Predicting Probable 
Roof Fall Areas in Advance of Mining by 
Geological Analysis. BuMines TPR 70, 
1973, 17 pp. 

25. Price, P. H. Geologic Considera- 
tions of Roof Support. Min. Congr. J., 
v. 35, Dec. 1949, pp. 45-58. 

26. Radcliffe, D. E. , and R. M. State- 
ham. Effects of Time Between Exposure 
and Support on Mine Roof Stability, Bear 
Coal Mine, Somerset, Colo. BuMines RI 
8298, 1978, 13 pp. 

27. . Long-Term Response of 

Coal Mine Roof to Time Lapse Between 
Exposure and Support. BuMines TPR 110, 
1980, 12 pp. 



28. Scheibner, B. J. Geology of the 
Single-Entry Project at Sunnyside Coal 
Mines 1 and 2, Sunnyside, Utah. BuMines 
RI 8402, 1979, 105 pp. 

29. Stahl, R. L. Guide to Geologic 
Features Affecting Coal Mine Roof. MSHA 
(U.S. Dep. Labor), IR 1101, 1979, 18 pp. 

30. Stateham, R. M. , and D. E. Rad- 
cliffe. Humidity: A Cyclic Effect in 
Coal Mine Roof Stability. BuMines RI 
8291, 1978, 19 pp. 



31. 



Roof Stability Studies in 



the Bear Mine, Somerset, Colorado; A Case 
History. Ch. in Ground Control in Room 
and Pillar Mining, ed. by Y. P. Chugh. 
Soc. of Min. Eng. AIME, Aug. 1982, 
pp. 41-51. 

32. . Time Variations in Coal 

Mine Roof Fall Rates. Cycles Mag., 
v. 29, No. 9, 1979, pp. 197-205. 

33. Stingelin, R. W. , J. R. Kern, and 
S. L. Morgan. Pre-Mining Identification 
of Hazards Associated with Coal Mine Roof 
Measures (contract J0177038, HRB-Singer, 
Inc.). BuMines OFR 167-81, 1981, 208 
pp.; NTIS PB 82-140344. 

34. Wier, C. E. Factors Affecting 
Coal Roof Rock in Sullivan County, Indi- 
ana. Proc. Acad. Sci. (Indianapolis), 
for 1969, v. 79, pp. 263-269. 



aU.S. CPO: 1981-705-020/5019 



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